When readdir runs out of entries in the directory it returns an undefined value in scalar context.

Update:...but that won't explain why you get an initialization error.

For example

#!/usr/bin/perl

use warnings;
use strict;

opendir(DIR_A, './testdir') or die $!;

while (my $job = readdir(DIR_A)) {
 print "$job\n";
 if ($job =~ /something/) {
         print "Something was seen\n";
 }
}
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prints

.
..
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when testdir is empty, and prints

.
..
something.pl
Something was seen
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when testdir contains the file something.pl.